#include <bits/stdc++.h>
using namespace std;
char FindNotRepeatingChar(string& a)
{
    map<char,int> m;
    for(auto i:a)
    {
        if(m.count(i) == 0)
            m.insert({i,1});
        else
            ++m.at(i);
    }
    for(auto i:m)
    {
        if(i.second == 1)
            return i.first;
    }
    return NULL;
}
int main() {
    char c;
    string a = "aabbcedhehdk";
    cout<<FindNotRepeatingChar(a);
    return 0;
}